Polyethylene composition having high swell ratio

ABSTRACT

Polyethylene composition with improved swell ratio and mechanical properties, particularly suited for preparing blow-molded articles, said composition having the following features: 
     1) density from 0.945 to less than 0.952 g/cm 3 ; 
     2) ratio MIF/MIP from 15 to 30; 
     3) Shear-Induced Crystallization Index SIC from 2.5 to 5.5.

This application is the U.S. National Phase of PCT InternationalApplication PCT/EP2013/072000, filed Oct. 22, 2013, claiming benefit ofpriority to European Patent Application No. 12189392.9, filed Oct. 22,2012, European Patent Application No. 12194530.7, filed Nov. 28, 2012,and claiming benefit of priority under 35 U.S.C. §119(e) of U.S.Provisional Application No. 61/730,925, filed Nov. 28, 2012, thecontents of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The present invention provides a polyethylene composition suitable forpreparing various kinds of formed articles. In particular, due to itsenhanced processability, high die-swell with high-quality surface anddimension stability of final article, environmental stress crackingresistance (FNCT) and impact resistance, the present composition issuitable for making extrusion blow-moulded hollow articles, such asdrums, containers and gasoline storage tanks.

The present invention also relates to a multi-stage polymerizationprocess for preparing the said polyethylene composition.

BACKGROUND OF THE INVENTION

An additional and important advantage of the polyethylene composition ofthe present invention is that it can be melt-processed at unusually highshear rate values, which mean high processing speeds and/or reducedmelt-processing temperatures, without encountering flow-instabilitieswhich generally produce unacceptable defects in the formed articles(e.g. shark skin or melt-fracture), even in the absence of processingaids.

Moreover, the fast crystallization kinetics of the present composition,which provides a critical contribution to its superior process-ability,also provides an unusually reduced shrinkage of the formed articles,thus allowing achieving a remarkable dimensional stability. Thus thecomposition of the present invention provides an unmatched balance ofmechanical properties and process-ability with respect to the knownpolyethylene compositions for the same use, as disclosed in particularin U.S. Pat. No. 6,201,078.

In fact, the polymers disclosed in U.S. Pat. No. 6,201,078 achieve arelatively low balance of swell ratio and environmental stress crackingresistance, as shown in the examples.

The problem of achieving a high impact resistance, reducing theflow-instabilities and improving the dimensional stability (loweringshrinkage) is not mentioned in such document.

SUMMARY OF THE INVENTION

Thus the present invention provides a polyethylene composition havingthe following features:

-   -   1) density from 0.945 to less than 0.952 g/cm³, preferably from        0.948 to 0.951 g/cm³, determined according to ISO 1183 at 23°        C.;    -   2) ratio MIF/MIP from 15 to 30, in particular from 17 to 29,        where MIF is the melt flow index at 190° C. with a load of 21.60        kg, and MIP is the melt flow index at 190° C. with a load of 5        kg, both determined according to ISO 1133;    -   3) SIC Index from 2.5 to 5.5, preferably from 2.5 to 4.5, more        preferably from 3.2 to 3.9;    -   wherein the SIC Index is the Shear-Induced Crystallization        Index, determined according to the following relation:        SIC Index=(t _(onset,SIC@1000) ×t _(onset,quiescent))/(HLMI)*100    -   where t_(onset,SIC)@1000 is measured in seconds and is the time        required for a crystallization onset under shear rate of 1000        s⁻¹, the t_(onset, quiescent) is measured in seconds and is the        crystallization onset time at temperature of 125° C. under no        shear, determined in isothermal mode by differential scanning        calorimetry (DSC); HLMI is the melt flow index determined at        190° C. with a load of 21.6 kg, according to ISO 1133.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood with reference to the followingdescription and appended claims, and accompanying drawing FIGURE where:

The drawing is an illustrative embodiment of a simplified process-flowdiagram of two serially connected gas-phase reactors (1 and 32/33)comprising lines 9, 10, 11, 14, 45, 46, 52 and 54 and a condenser 49suitable for use in accordance with various embodiments of ethylenepolymerization processes disclosed herein to produce various embodimentsof the polyethylene compositions disclosed herein.

It should be understood that the various embodiments are not limited tothe arrangements and instrumentality shown in the drawing FIGURE.

DETAILED DESCRIPTION OF THE INVENTION

The expression “polyethylene composition” is intended to embrace, asalternatives, both a single ethylene polymer and an ethylene polymercomposition, in particular a composition of two or more ethylene polymercomponents, preferably with different molecular weights, suchcomposition being also called “bimodal” or “multimodal” polymer in therelevant art. Typically the polyethylene composition of the presentinvention consists of or comprises one or more ethylene copolymers.

All the features herein defined, comprising the previously definedfeatures 1) to 3), are referred to the said ethylene polymer or ethylenepolymer composition. The addition of other components, like theadditives normally employed in the art, can modify one or more of saidfeatures.

The ratio MIF/MIP provides a rheological measure of molecular weightdistribution.

Another measure of the molecular weight distribution is provided by theratio Mw/Mn, where Mw is the weight average molar mass and Mn is thenumber average molar mass, both measured by GPC (Gel PermeationChromatography), as explained in the examples. Preferred Mw/Mn valuesfor the polyethylene composition of the present invention range from 20to 30.

Moreover the polyethylene composition of the present invention haspreferably at least one of the following additional features.

-   -   Mw equal to or greater than 250000 g/mol, more preferably equal        to or greater than 280000 g/mol, in particular equal to or        greater than 300000 g/mol;    -   Long Chain Branching index (LCB) determined as described in the        examples, equal to or greater than 0.70, more preferably equal        to or greater than 0.72, in particular equal to or greater than        0.80;    -   MIP: 0.05-0.5 g/10 min.;    -   MIF: 1-15 g/10 min;    -   Comonomer content equal to or less than 1% by weight, in        particular from 0.05 to 1% by weight, with respect to the total        weight of the composition.

The comonomer or comonomers present in the ethylene copolymers aregenerally selected from olefins having formula CH₂═CHR wherein R is analkyl radical, linear or branched, having from 1 to 10 carbon atoms.

Specific examples are propylene, butene-1, pentene-1,4-methylpentene-1,hexene-1, octene-1 and decene-1. A particularly preferred comonomer ishexene-1.

In particular, in a preferred embodiment, the present compositioncomprises:

-   A) 30-50% by weight of an ethylene homopolymer or copolymer (the    homopolymer being preferred) with density equal to or greater than    0.960 g/cm³ and melt flow index MIE at 190° C. with a load of 2.16    kg, according to ISO 1133, of 10-35 g/10 min.;-   B) 50-70% by weight of an ethylene copolymer having a MIE value    lower than the MIE value of A), preferably lower than 0.5 g/10 min.

The above percent amounts are given with respect to the total weight ofA)+B).

The amount of comonomer in B) is preferably from 0.1 to 2% by weight,with respect to the total weight of B).

As previously said, the present polyethylene composition can beadvantageously used in the preparation of extrusion blow-moulded hollowarticles, in particular large blow moulded articles such asopen-top-drums (OTD) or industrial-bulk-containers (IBC), thanks to itsvaluable mechanical properties.

In fact it is preferably characterized by the following properties.

-   -   FNCT equal to or greater than 10 hours, more preferably equal to        or greater than 100 hours, in particular equal to or greater        than 150 hours, measured at 4 MPa, 80° C.;    -   Notch Tensile Impact (−30° C.) equal to or greater than 100        kJ/m²;    -   Critical shear-rate for shark skin (190° C.) equal to or greater        than 250 s⁻¹;    -   Die swell-ratio equal to or greater than 150%;    -   Shrinkage at 1500 s⁻¹ (190° C.) equal to or smaller than 17%.

The details of the test methods are given in the examples.

In particular the shark skin test (critical shear rate for shark skin)provides an indication of the shear rate at which flow-instabilitiesstart due to pressure oscillations, thus of the melt processingconditions, and consequently of the extrusion throughput, at whichirregularities on the surface of the extruded piece become visible. Suchirregularities strongly reduce surface gloss and smoothness, thuslowering the quality of the extruded article to an unacceptable level.

As previously mentioned, the polyethylene composition of the presentinvention can be melt-processed at surprisingly high values of shearrate, still without undergoing pressure oscillations andflow-instabilities.

While no necessary limitation is known to exist in principle on the kindof polymerization processes and catalysts to be used, it has been foundthat the polyethylene composition of the present invention can beprepared by a gas phase polymerization process in the presence of aZiegler-Natta catalyst.

A Ziegler-Natta catalyst comprises the product of the reaction of anorganometallic compound of group 1, 2 or 13 of the Periodic Table ofelements with a transition metal compound of groups 4 to 10 of thePeriodic Table of Elements (new notation). In particular, the transitionmetal compound can be selected among compounds of Ti, V, Zr, Cr and Hfand is preferably supported on MgCl₂.

Particularly preferred catalysts comprise the product of the reaction ofsaid organometallic compound of group 1, 2 or 13 of the Periodic Tableof elements, with a solid catalyst component comprising a Ti compoundand an electron donor compound (ED) supported on MgCl₂.

Preferred organometallic compounds are the organo-Al compounds.

Thus in a preferred embodiment, the polyethylene composition of thepresent invention is obtainable by using a Ziegler-Natta polymerizationcatalyst, more preferably a Ziegler-Natta catalyst supported on MgCl₂,even more preferably a Ziegler-Natta catalyst comprising the product ofreaction of:

-   a) a solid catalyst component comprising a Ti compound and an    electron donor compound ED supported on MgCl₂;-   b) an organo-Al compound; and optionally-   c) an external electron donor compound ED_(ext).

Preferably in component a) the ED/Ti molar ratio ranges from 1.5 to 3.5and the Mg/Ti molar ratio is higher than 5.5, in particular from 6 to80.

Among suitable titanium compounds are the tetrahalides or the compoundsof formula TiX_(n)(OR¹)_(4-n), where 0≦n≦3, X is halogen, preferablychlorine, and R¹ is C₁-C₁₀ hydrocarbon group. The titanium tetrachlorideis the preferred compound.

The ED compound is generally selected from alcohol, ketones, amines,amides, nitriles, alkoxysilanes, aliphatic ethers, and esters ofaliphatic carboxylic acids.

Preferably the ED compound is selected among, amides, esters andalkoxysilanes.

Excellent results have been obtained with the use of esters which arethus particularly preferred as ED compound. Specific examples of estersare the alkyl esters of C1-C20 aliphatic carboxylic acids and inparticular C1-C8 alkyl esters of aliphatic mono carboxylic acids such asethylacetate, methyl formiate, ethylformiate, methylacetate,propylacetate, propylacetate, n-butylacetate, i-butylacetate. Moreover,are also preferred the aliphatic ethers and particularly the C2-C20aliphatic ethers, such as tetrahydrofurane (THF) or dioxane.

In the said solid catalyst component the MgCl₂ is the basic support,even if minor amount of additional carriers can be used. The MgCl₂ canbe used as such or obtained from Mg compounds used as precursors thatcan be transformed into MgCl₂ by the reaction with halogenatingcompounds. Particularly preferred is the use of MgCl₂ in active formwhich is widely known from the patent literature as a support forZiegler-Natta catalysts. U.S. Pat. No. 4,298,718 and U.S. Pat. No.4,495,338 were the first to describe the use of these compounds inZiegler-Natta catalysis. It is known from these patents that themagnesium dihalides in active form used as support or co-support incomponents of catalysts for the polymerization of olefins arecharacterized by X-ray spectra in which the most intense diffractionline that appears in the ASTM-card reference of the spectrum of thenon-active halide is diminished in intensity and broadened. In the X-rayspectra of preferred magnesium dihalides in active form said mostintense line is diminished in intensity and replaced by a halo whosemaximum intensity is displaced towards lower angles relative to that ofthe most intense line. Particularly suitable for the preparation of thepolyethylene composition of the present invention are the catalystswherein the solid catalyst component a) is obtained by first contactingthe titanium compound with the MgCl₂, or a precursor Mg compound,optionally in the presence of an inert medium, thus preparing anintermediate product a′) containing a titanium compound supported onMgCl₂, which intermediate product a′) is then contacted with the EDcompound which is added to the reaction mixture alone or in a mixturewith other compounds in which it represents the main component,optionally in the presence of an inert medium.

With the term “main component” we intend that the said ED compound mustbe the main component in terms of molar amount, with respect to theother possible compounds excluded inert solvents or diluents used tohandle the contact mixture. The ED treated product can then be subjectto washings with the proper solvents in order to recover the finalproduct. If needed, the treatment with the ED compound desired can berepeated one or more times.

As previously mentioned, a precursor of MgCl₂ can be used as startingessential Mg compound. This can be selected for example among Mgcompound of formula MgR′₂ where the R′ groups can be independentlyC1-C20 hydrocarbon groups optionally substituted, OR groups, OCORgroups, chlorine, in which R is a C1-C20 hydrocarbon groups optionallysubstituted, with the obvious proviso that the R′ groups are notsimultaneously chlorine. Also suitable as precursors are the Lewisadducts between MgCl₂ and suitable Lewis bases. A particular andpreferred class being constituted by the MgCl₂ (R″OH)_(m) adducts inwhich R″ groups are C1-C20 hydrocarbon groups, preferably C1-C10 alkylgroups, and m is from 0.1 to 6, preferably from 0.5 to 3 and morepreferably from 0.5 to 2. Adducts of this type can generally be obtainedby mixing alcohol and MgCl₂ in the presence of an inert hydrocarbonimmiscible with the adduct, operating under stirring conditions at themelting temperature of the adduct (100-130° C.). Then, the emulsion isquickly quenched, thereby causing the solidification of the adduct inform of spherical particles. Representative methods for the preparationof these spherical adducts are reported for example in U.S. Pat. No.4,469,648, U.S. Pat. No. 4,399,054, and WO98/44009. Another useablemethod for the spherulization is the spray cooling described for examplein U.S. Pat. Nos. 5,100,849 and 4,829,034.

Particularly interesting are the MgCl₂.(EtOH)_(m) adducts in which m isfrom 0.15 to 1.7 obtained subjecting the adducts with a higher alcoholcontent to a thermal dealcoholation process carried out in nitrogen flowat temperatures comprised between 50 and 150° C. until the alcoholcontent is reduced to the above value. A process of this type isdescribed in EP 395083.

The dealcoholation can also be carried out chemically by contacting theadduct with compounds capable to react with the alcohol groups.

Generally these dealcoholated adducts are also characterized by aporosity (measured by mercury method) due to pores with radius up to 0.1μm ranging from 0.15 to 2.5 cm³/g preferably from 0.25 to 1.5 cm³/g.

It is preferred that the dealcoholation reaction is carried outsimultaneously with the step of reaction involving the use of a titaniumcompound. Accordingly, these adducts are reacted with theTiX_(n)(OR¹)_(4-n) compound (or possibly mixtures thereof) mentionedabove which is preferably titanium tetrachloride. The reaction with theTi compound can be carried out by suspending the adduct in TiCl₄(generally cold). The mixture is heated up to temperatures ranging from80-130° C. and kept at this temperature for 0.5-2 hours. The treatmentwith the titanium compound can be carried out one or more times.Preferably it is repeated twice. It can also be carried out in thepresence of an electron donor compound as those mentioned above. At theend of the process the solid is recovered by separation of thesuspension via the conventional methods (such as settling and removingof the liquid, filtration, centrifugation) and can be subject towashings with solvents. Although the washings are typically carried outwith inert hydrocarbon liquids, it is also possible to use more polarsolvents (having for example a higher dielectric constant) such ashalogenated hydrocarbons.

As mentioned above, the intermediate product is then brought intocontact with the ED compound under conditions able to fix on the solidan effective amount of donor. Due to the high versatility of thismethod, the amount of donor used can widely vary. As an example, it canbe used in molar ratio with respect to the Ti content in theintermediate product ranging from 0.5 to 20 and preferably from 1 to 10.Although not strictly required the contact is typically carried out in aliquid medium such as a liquid hydrocarbon. The temperature at which thecontact takes place can vary depending on the nature of the reagents.Generally it is comprised in the range from −10° to 150° C. andpreferably from 0° to 120° C. It is plane that temperatures causing thedecomposition or degradation of any specific reagents should be avoidedeven if they fall within the generally suitable range. Also the time ofthe treatment can vary in dependence of other conditions such as natureof the reagents, temperature, concentration etc. As a general indicationthis contact step can last from 10 minutes to 10 hours more frequentlyfrom 0.5 to 5 hours. If desired, in order to further increase the finaldonor content, this step can be repeated one or more times. At the endof this step the solid is recovered by separation of the suspension viathe conventional methods (such as settling and removing of the liquid,filtration, centrifugation) and can be subject to washings withsolvents. Although the washings are typically carried out with inerthydrocarbon liquids, it is also possible to use more polar solvents(having for example a higher dielectric constant) such as halogenated oroxygenated hydrocarbons.

As previously mentioned, the said solid catalyst component is convertedinto catalysts for the polymerization of olefins by reacting it,according to known methods, with an organometallic compound of group 1,2 or 13 of the Periodic Table of elements, in particular with anAl-alkyl compound.

The alkyl-Al compound is preferably chosen among the trialkyl aluminumcompounds such as for example triethylaluminum, triisobutylaluminum,tri-n-butylaluminum, tri-n-hexylaluminum, tri-n-octylaluminum. It isalso possible to use alkylaluminum halides, alkylaluminum hydrides oralkylaluminum sesquichlorides such as AlEt₂Cl and Al₂Et₃Cl₃ optionallyin mixture with said trialkyl aluminum compounds.

The external electron donor compound ED_(ext) optionally used to preparethe said Ziegler-Natta catalysts can be equal to or different from theED used in the solid catalyst component a). Preferably it is selectedfrom the group consisting of ethers, esters, amines, ketones, nitriles,silanes and their mixtures. In particular it can advantageously beselected from the C2-C20 aliphatic ethers and in particulars cyclicethers preferably having 3-5 carbon atoms such as tetrahydrofurane anddioxane.

Specific examples of the above described Ziegler-Natta catalysts and ofmethods for their preparation are provided in WO2004106388. However, thetherein described prepolymerization of the solid catalyst componentcontaining the Ti compound and the electron donor compound ED (solidcatalyst component a)) is not comprised in the preferred embodimentsaccording to the present invention.

In particular, the polyethylene composition of the present invention isobtainable by a process wherein all the polymerization steps are carriedout in the presence of the said catalyst.

In fact it has been found that by using the above describedpolymerization catalyst, the polyethylene composition of the presentinvention can be prepared in a process comprising the following steps,in any mutual order:

-   a) polymerizing ethylene, optionally together with one or more    comonomers, in a gas-phase reactor in the presence of hydrogen;-   b) copolymerizing ethylene with one or more comonomers in another    gas-phase reactor in the presence of an amount of hydrogen less than    step a);    where in at least one of said gas-phase reactors the growing polymer    particles flow upward through a first polymerization zone (riser)    under fast fluidization or transport conditions, leave said riser    and enter a second polymerization zone (downcomer) through which    they flow downward under the action of gravity, leave said downcomer    and are reintroduced into the riser, thus establishing a circulation    of polymer between said two polymerization zones. In the first    polymerization zone (riser), fast fluidization conditions are    established by feeding a gas mixture comprising one or more olefins    (ethylene and comonomers) at a velocity higher than the transport    velocity of the polymer particles. The velocity of said gas mixture    is preferably comprised between 0.5 and 15 m/s, more preferably    between 0.8 and 5 m/s. The terms “transport velocity” and “fast    fluidization conditions” are well known in the art; for a definition    thereof, see, for example, “D. Geldart, Gas Fluidisation Technology,    page 155 et seq., J. Wiley & Sons Ltd., 1986”.

In the second polymerization zone (downcomer), the polymer particlesflow under the action of gravity in a densified form, so that highvalues of density of the solid are reached (mass of polymer per volumeof reactor), which approach the bulk density of the polymer.

In other words, the polymer flows vertically down through the downcomerin a plug flow (packed flow mode), so that only small quantities of gasare entrained between the polymer particles.

Such process allows to obtain from step a) an ethylene polymer with amolecular weight lower than the ethylene copolymer obtained from stepb).

Preferably, the polymerization of ethylene to produce a relatively lowmolecular weight ethylene polymer (step a) is performed upstream thecopolymerization of ethylene with a comonomer to produce a relativelyhigh molecular weight ethylene copolymer (step b). To this aim, in stepa) a gaseous mixture comprising ethylene, hydrogen and an inert gas isfed to a first gas-phase reactor, preferably a gas-phase fluidized bedreactor. The polymerization is carried out in the presence of thepreviously described Ziegler-Natta catalyst. Preferably, no comonomer isfed to said first gas phase reactor and a highly crystalline ethylenehomopolymer is obtained in step a). However, a minimal amount ofcomonomer may be fed with the proviso that the degree ofcopolymerization in step a) is limited so that the density of theethylene polymer obtained in step a) is not less than 0.960 g/cm³.

Hydrogen is fed in an amount depending on the specific catalyst usedand, in any case, suitable to obtain in step a) an ethylene polymer witha melt flow index MIE from 10 to 35 g/10 min. In order to obtain theabove MIE range, in step a) the hydrogen/ethylene molar ratio isindicatively from 0.5 to 2, the amount of ethylene monomer being from 5to 50% by volume, preferably from 5 to 30% by volume, based on the totalvolume of gas present in the polymerization reactor. The remainingportion of the feeding mixture is represented by inert gases and one ormore comonomers, if any. Inert gases which are necessary to dissipatethe heat generated by the polymerization reaction are convenientlyselected from nitrogen or saturated hydrocarbons, the most preferredbeing propane.

The operating temperature in the reactor of step a) is selected between50 and 120° C., preferably between 65 and 100° C., while the operatingpressure is between 0.5 and 10 MPa, preferably between 2.0 and 3.5 MPa.

In a preferred embodiment, the ethylene polymer obtained in step a)represents from 30 to 50% by weight of the total ethylene polymerproduced in the overall process, i.e. in the first and second seriallyconnected reactors.

The ethylene polymer coming from step a) and the entrained gas are thenpassed through a solid/gas separation step, in order to prevent thegaseous mixture coming from the first polymerization reactor fromentering the reactor of step b) (second gas-phase polymerizationreactor). Said gaseous mixture can be recycled back to the firstpolymerization reactor, while the separated ethylene polymer is fed tothe reactor of step b). A suitable point of feeding of the polymer intothe second reactor is on the connecting part between the downcomer andthe riser, wherein the solid concentration is particularly low, so thatthe flow conditions are not negatively affected.

The operating temperature in step b) is in the range of 65 to 95° C.,and the pressure is in the range of 1.5 to 4.0 MPa. The second gas-phasereactor is aimed to produce a relatively high molecular weight ethylenecopolymer by copolymerizing ethylene with one or more comonomers.Furthermore, in order to broaden the molecular weight distribution ofthe final ethylene polymer, the reactor of step b) can be convenientlyoperated by establishing different conditions of monomers and hydrogenconcentration within the riser and the downcomer.

To this purpose, in step b) the gas mixture entraining the polymerparticles and coming from the riser can be partially or totallyprevented from entering the downcomer, so to obtain two different gascomposition zones. This can be achieved by feeding a gas and/or a liquidmixture into the downcomer through a line placed at a suitable point ofthe downcomer, preferably in the upper part thereof. Said gas and/orliquid mixture should have a suitable composition, different from thatof the gas mixture present in the riser. The flow of said gas and/orliquid mixture can be regulated so that an upward flow of gascounter-current to the flow of the polymer particles is generated,particularly at the top thereof, acting as a barrier to the gas mixtureentrained among the polymer particles coming from the riser. Inparticular, it is advantageous to feed a mixture with low content ofhydrogen in order to produce the higher molecular weight polymerfraction in the downcomer. One or more comonomers can be fed to thedowncomer of step b), optionally together with ethylene, propane orother inert gases.

The hydrogen/ethylene molar ratio in the downcomer of step b) iscomprised between 0.005 and 0.2, the ethylene concentration beingcomprised from 1 to 20%, preferably 3-10%, by volume, the comonomerconcentration being comprised from 0.2 to 1% by volume, based on thetotal volume of gas present in said downcomer. The rest is propane orsimilar inert gases. Since a very low molar concentration of hydrogen ispresent in the downcomer, by carrying out the process of the presentinvention is possible to bond a relatively high amount of comonomer tothe high molecular weight polyethylene fraction.

The polymer particles coming from the downcomer are reintroduced in theriser of step b).

Since the polymer particles keep reacting and no more comonomer is fedto the riser, the concentration of said comonomer drops to a range of0.1 to 0.5% by volume, based on the total volume of gas present in saidriser. In practice, the comonomer content is controlled in order toobtain the desired density of the final polyethylene. In the riser ofstep b) the hydrogen/ethylene molar ratio is in the range of 0.05 to0.3, the ethylene concentration being comprised between 5 and 15% byvolume based on the total volume of gas present in said riser. The restis propane or other inert gases.

More details on the above described polymerization process are providedin WO9412568.

EXAMPLES

The following examples are given to illustrate, without limiting, thepresent invention.

Unless differently stated, the following test methods are used todetermine the properties reported in the detailed description and in theexamples.

Density

-   -   Determined according to ISO 1183 at 23° C.

Molecular Weight Distribution Determination

-   -   The determination of the molar mass distributions and the means        Mn, Mw and Mw/Mn derived therefrom was carried out by        high-temperature gel permeation chromatography using a method        described in ISO 16014-1, -2, -4, issues of 2003. The specifics        according to the mentioned ISO standards are as follows: Solvent        1,2,4-trichlorobenzene (TCB), temperature of apparatus and        solutions 135° C. and as concentration detector a PolymerChar        (Valencia, Paterna 46980, Spain) IR-4 infrared detector, capable        for use with TCB. A WATERS Alliance 2000 equipped with the        following pre-column SHODEX UT-G and separation columns SHODEX        UT 806 M (3×) and SHODEX UT 807 (Showa Denko Europe GmbH,        Konrad-Zuse-Platz 4, 81829 Muenchen, Germany) connected in        series was used. The solvent was vacuum distilled under Nitrogen        and was stabilized with 0.025% by weight of        2,6-di-tert-butyl-4-methylphenol. The flowrate used was 1        ml/min, the injection was 500 μl and polymer concentration was        in the range of 0.01%<conc.<0.05% w/w. The molecular weight        calibration was established by using monodisperse polystyrene        (PS) standards from Polymer Laboratories (now Agilent        Technologies, Herrenberger Str. 130, 71034 Boeblingen, Germany))        in the range from 580 g/mol up to 11600000 g/mol and        additionally with Hexadecane. The calibration curve was then        adapted to Polyethylene (PE) by means of the Universal        Calibration method (Benoit H., Rempp P. and Grubisic Z., & in J.        Polymer Sci., Phys. Ed., 5, 753(1967)). The Mark-Houwing        parameters used herefore were for PS: k_(PS)=0.000121 dl/g,        α_(PS)=0.706 and for PE k_(PE)=0.000406 dl/g, α_(PE)=0.725,        valid in TCB at 135° C. Data recording, calibration and        calculation was carried out using NTGPC_Control_V6.02.03 and        NTGPC_V6.4.24 (hs GmbH, Hauptstraβe 36, D-55437        Ober-Hilbersheim, Germany) respectively.

Shear-Induced Crystallization Test

-   -   This method is utilized to determine the onset time of        shear-induced crystallization (SIC) of the polymer,        t_(onset,SIC). Samples are melt-pressed at 210° C., 4 min, under        200 bar in a lab press to 1 mm thick-plaques. Disc specimens are        cut-out with a diameter of 25 mm. The samples are inserted in        the plate-plate oscillatory-shear rheometer. A Physica MCR 301        rotational rheometer from AntonPaar is used.    -   The sample is then molten inside the test-geometry at 190° C.        for 4 min, cooled down with ˜10K/min to the test temperature,        T=125° C., and annealed for 5 min. Consequently, steady-shear        under constant shear rate is applied and the shear viscosity is        monitored as a function of time. The experiment is repeated        applying each time a different shear-rate ranging from 0.05 to        0.5 s⁻¹. The onset time for SIC, t_(onset,SIC), is taken at the        point where the viscosity has increased at 50% of its        steady-state value η@125° C. The steady-state value is the        average of the steady-shear melt viscosity measured at the        specific temperature.    -   The plot of log t_(onset,SIC) vs. log shear-rate provides a        linear function (of type y=Ax+B) which is extrapolated to a        shear rate of 1000 s⁻¹ (process-relevant) to determine the value        of t_(onset,SIC)@1000.    -   The SIC Index is then calculated according to the following        relation:        SIC Index=(t _(onset,SIC)*1000×t _(onset,quiescent))/(HLMI)    -   The t_(onset, quiescent) (in sec) is the crystallization onset        at temperature of 125° C. under quiescent conditions, i.e. no        shear, measured in isothermal mode in a        differential-scanning-calorimetry apparatus, DSC, as hereinafter        explained.    -   HLMI is the melt flow index (g/10 min) measured at T=190° C.        with 21.6 kg load, according to ISO 1133.    -   The same protocol is described in the following documents.

-   I. Vittorias, Correlation among structure, processing and product    properties, Würzburger Tage 2010, Wolfgang Kunze TA Instruments,    Germany.

-   Wo D L, Tanner R I (2010), The impact of blue organic and inorganic    pigments on the crystallization and rheological properties of    isotactic polypropylene, Rheol. Acta 49, 75.

-   Derakhshandeh M., Hatzikiriakos S. G., Flow-induced crystallization    of high-density polyethylene: the effects of shear and uniaxial    extension, Rheol. Acta, 51, 315-327, 2012.

Isothermal DSC

-   -   The t_(onset,quiescent), the onset time when no deformation is        applied at 125° C., is determined by the iso-DSC (isothermal        Differential Scanning calorimetry) method. It is measured at        125° C. in a TA Instruments Q2000 DSC apparatus. The        determination of the t_(onset,quiescent) is performed utilizing        the commercially available software TA Universal Analysis 2000.        The sample preparation and set-up follows the DIN EN ISO        11357-1:2009 and ISO 11357-3:1999.

Melt Flow Index

-   -   Determined according to ISO 1133 at 190° C. with the specified        load.

Long Chain Branching Index (LCB)

-   -   The LCB index corresponds to the branching factor g′, measured        for a molecular weight of 10⁶ g/mol. The branching factor g′,        which allows determining long-chain branches at high Mw, was        measured by Gel Permeation Chromatography (GPC) coupled with        Multi-Angle Laser-Light Scattering (MALLS), as described in the        following. The parameter g′ is the ratio of the measured mean        square radius of gyration to that of a linear polymer having the        same molecular weight. Linear molecules show g′ of 1, while        values less than 1 indicate the presence of LCB. Values of g′ as        a function of mol. weight, M, were calculated from the equation:        g′(M)=<Rg ²>_(sample,M) /<Rg ²>_(linear ref.,M)    -   where <Rg²>,M is the root-mean-square radius of gyration for the        fraction of mol. weight M.    -   The radius of gyration for each fraction eluted from the GPC (as        described above but with a flow-rate of 0.6 ml/min and a column        packed with 30 nm particles) is measured by analyzing the light        scattering at the different angles. Therefore, from this MALLS        setup it is possible to determine mol. weight M and        <Rg²>_(sample,M) and to define a g′ at a measured M=10⁶ g/mol.        The <Rg²>_(linear ref.,M) is calculated by the established        relation between radius-of-gyration and molecular weight for a        linear polymer in solution (Zimm and Stockmayer W H 1949)) and        confirmed by measuring a linear PE reference with the same        apparatus and methodology described.    -   The same protocol is described in the following documents.

-   Zimm B H, Stockmayer W H (1949) The dimensions of chain molecules    containing branches and rings. J Chem Phys 17

-   Rubinstein M., Colby R H. (2003), Polymer Physics, Oxford University    Press

Comonomer Content

-   -   The comonomer content is determined by means of IR in accordance        with ASTM D 6248 98, using an FT-IR spectrometer Tensor 27 from        Bruker, calibrated with a chemometric model for determining        ethyl- or butyl-side-chains in PE for butene or hexene as        comonomer, respectively.

Swell Ratio

-   -   The Swell-ratio of the studied polymers is measured utilizing a        capillary rheometer, Göttfert Rheotester2000 and Rheograph25, at        T=190° C., equipped with a commercial 30/2/2/20 die (total        length 30 mm, Active length=2 mm, diameter=2 mm, L/D=2/2 and 20°        entrance angle) and an optical device (laser-diod from Göttfert)        for measuring the extruded strand thickness. Sample is molten in        the capillary barrel at 190° C. for 6 min and extruded with a        piston velocity corresponding to a resulting shear-rate at the        die of 1440 s⁻¹. The extrudate is cut (by an automatic cutting        device from Göttfert) at a distance of 150 mm from the die-exit,        at the moment the piston reaches a position of 96 mm from the        die-inlet. The extrudate diameter is measured with the        laser-diod at a distance of 78 mm from the die-exit, as a        function of time. The maximum value corresponds to the        D_(extrudate). The swell-ratio is determined from the        calculation: SR═(D_(extrudate)−D_(die))100%/D_(die)    -   where D_(die) is the corresponding diameter at the die exit,        measured with the laser-diod.

Shrinkage@1500 s⁻¹ (Shrinkage Lab-Test)

-   -   This method is applied in order to determine the shrinkage of        the final product of polyethylene after melt-extrusion, or in        other words the dimension stability potential of a grade. The        method is recommended for homogeneous PE in granulate form.        Samples in powder can be measured, only after stabilizing and        melt-homogenization (typically in a lab-plasticizer-kneader).        However in the latter case, one should expect a significant        effect on the results, mainly due to the fact that the sample is        more sensitive to degradation and air-bubbles in the extrudate.    -   The samples in granulate form can be used directly and        approximately 20 g of sample are needed for filling the        capillary barrel. The utilized capillary rheometer is a Göttfert        Rheotester 2000, with a 15 mm diameter barrel and applicable        pressure range 0-2000 bar, temperatures 25-400° C., equipped        with a 30/2/2/20 die, with total length 30 mm, L/D=2/2 and 20°        entrance angle. The recommended test temperature for        polyethylene is 210° C.    -   The piston speed is set in order to have the required apparent        shear rate at the die exit. The test is performed at shear rates        50 s⁻¹, 1000 s⁻¹, 1500 s⁻¹ and 2500 s⁻¹.    -   The extrudate is marked and pieces of 40 mm length each are        punched/stamped, while still in the molten-state, and left to        cool at room temperature. At least 3 parts of 40 mm must be        marked in this way. A pinch-off metal-tool is utilized to stamp        the extrudate after the die-exit in the parts to be measured,        with a length of 40 mm (initial length for each part, L_(i,0))        and typically 10 mm wide.    -   The whole extrudate is cut and left on a lab table to        crystallize and cool down at room temperature for at least 15        min. The parts are cut at the marks and measured in length. The        resulting length, L_(i), in mm is recorded for each part i and        averaged for 4 parts.

${Shrinkage}_{i} = {{\frac{L_{0} - L_{i}}{L_{0}} \times 100\%} = {\frac{\Delta\; L_{i}}{L_{0}} \times 100\%}}$${Shrinkage}_{average} = \frac{\sum{Shinkage}_{i}}{i}$

-   -   The procedure is undertaken for each applied shear-rate and the        measurement of shrinkage for each shear-rate is repeated at        least two times.    -   Remark: Deviations of the shrinkage along the extrudate length        are expected, i.e. due to varying cooling time after exiting the        die for each part and sagging (the punched part leaving last the        die will be less time exposed to room temperature and        “stretched” due to the extrudate weight).

Critical Shear Rate for Sharkskin (Sharkskin Test)

-   -   The sharkskin test is a method to quantify the        flow-instabilities and surface defects occurring during        extrusion of polymer melts. Specifically, the commercial        sharkskin-option with the Rheotester2000 capillary rheometer        from Göttfert is used. The sharkskin-option is a slit-die of        30×3×0.3 mm with three pressure transducers distributed along        the die (at die-entry, middle and before die-exit). The pressure        is recorded and analyzed (Fourier-transformation) using the        available commercial Göttfert WebRheo software.    -   The polymer is extruded at 190° C. applying the following        shear-rates in this specific order:        100-150-200-250-300-350-400-450-500 s⁻¹. The extrudate is        consequently visually inspected for surface defects. The        critical shear-rate for sharkskin instability is the applied        shear-rate for which the sharkskin instability first occurs        (high frequency pressure oscillations and visually detectable        periodic surface distortions).    -   The same protocol is described in the following documents.

-   Palza H., Naue I. F. C., Wilhelm M., Filipe S., Becker A., Sunder    J., Göttfert A., On-Line Detection of Polymer Melt Flow    Instabilities in a Capillary Rheometer, KGK. Kautschuk, Gummi,    Kunststoffe, 2010, vol. 63, no 10, pp. 456-461.

-   Susana Filipe, Iakovos Vittorias, Manfred Wilhelm, Experimental    Correlation between Mechanical Non-Linearity in LAOS Flow and    Capillary Flow Instabilities for Linear and Branched Commercial    Polyethylenes, Macromol. Mat. and Eng., Volume 293, Issue 1, pages    57-65, 2008.

-   Göttfert, A.; Sunder, J., AIP Conference Proceedings, Volume 1027,    pp. 1195-1197 (2008).

Notched Tensile Impact Test

-   -   The tensile-impact strength is determined using ISO 8256:2004        with type 1 double notched specimens according to method A. The        test specimens (4×10×80 mm) are cut form a compression molded        sheet which has been prepared according ISO 1872-2 requirements        (average cooling rate 15 K/min and high pressure during cooling        phase). The test specimens are notched on two sides with a 45°        V-notch. Depth is 2±0.1 mm and curvature radius on notch dip is        1.0±0.05 mm. The free length between grips is 30±2 mm. Before        measurement, all test specimens are conditioned at a constant        temperature of −30° C. over a period of from 2 to 3 hours. The        procedure for measurements of tensile impact strength including        energy correction following method A is described in ISO 8256.

Environmental Stress Cracking Resistance According to Full Notch CreepTest (FNCT)

-   -   The environmental stress cracking resistance of polymer samples        is determined in accordance to international standard ISO 16770        (FNCT) in aqueous surfactant solution. From the polymer sample a        compression moulded 10 mm thick sheet has been prepared. The        bars with squared cross section (10×10×100 mm) are notched using        a razor blade on four sides perpendicularly to the stress        direction. A notching device described in M. Fleissner in        Kunststoffe 77 (1987), pp. 45 is used for the sharp notch with a        depth of 1.6 mm. The load applied is calculated from tensile        force divided by the initial ligament area. Ligament area is the        remaining area=total cross-section area of specimen minus the        notch area. For FNCT specimen: 10×10 mm²—4 times of trapezoid        notch area=46.24 mm² (the remaining cross-section for the        failure process/crack propagation). The test specimen is loaded        with standard condition suggested by the ISO 16770 with constant        load of 4 MPa at 80° C. in a 2% (by weight) water solution of        non-ionic surfactant ARKOPAL N100. Time until rupture of test        specimen is detected.

Charpy aFM

-   -   Fracture toughness determination by an internal method on test        bars measuring 10×10×80 mm which had been sawn out of a        compression molded sheet with a thickness of 10 mm. Six of these        test bars are notched in the center using a razor blade in the        notching device mentioned above for FNCT. The notch depth is        1.6 mm. The measurement is carried out substantially in        accordance with the Charpy measurement method in accordance with        ISO 179-1, with modified test specimens and modified impact        geometry (distance between supports). All test specimens are        conditioned to the measurement temperature of 0° C. over a        period of from 2 to 3 hours. A test specimen is then placed        without delay onto the support of a pendulum impact tester in        accordance with ISO 179-1. The distance between the supports is        60 mm. The drop of the 2 J hammer is triggered, with the drop        angle being set to 160°, the pendulum length to 225 mm and the        impact velocity to 2.93 m/s. The fracture toughness value is        expressed in kJ/m² and is given by the quotient of the impact        energy consumed and the initial cross-sectional area at the        notch, aFM. Only values for complete fracture and hinge fracture        can be used here as the basis for a common meaning (see        suggestion by ISO 179-1).

Examples 1, 2 and Comparative Examples 1 and 2 Process Setup

In Examples 1-2 the process of the invention was carried out undercontinuous conditions in a plant comprising two serially connectedgas-phase reactors, as shown in the FIGURE.

Comparative Example 1 is carried out in the same plant under continuousconditions as well.

Example 1

-   -   The solid catalyst component was prepared as described in        Example 13 of WO2004106388. The AcOEt/Ti molar ratio was of 8.    -   Polymerization

7 g/h of the he solid catalyst component prepared as described abovewere fed, using 5 kg/h of liquid propane, to a precontacting apparatus,in which also a mixture of triisobuthylaluminum (TIBA) anddiethylaluminum chloride (DEAC) as well tetrahydrofuran (THF) weredosed. The weight ratio between TIBA and DEAC was 7:1. The weight ratiobetween aluminum alkyl and solid catalyst component was 10:1. The weightratio between aluminum alkyl and THF was 70. The precontacting step wascarried out under stirring at 50° C. with a total residence time of 70minutes.

The catalyst enters the first gas-phase polymerization reactor 1 of theFIGURE via line 10. In the first reactor ethylene was polymerized usingH₂ as molecular weight regulator and in the presence of propane as inertdiluent. 35 kg/h of ethylene and 62 g/h of hydrogen were fed to thefirst reactor via line 9. No comonomer was fed to the first reactor.

The polymerization was carried out at a temperature of 75° C. and at apressure of 2.5 MPa. The polymer obtained in the first reactor wasdiscontinuously discharged via line 11, separated from the gas into thegas/solid separator 12, and reintroduced into the second gas-phasereactor via line 14.

The polymer produced in the first reactor had a melt index MIE of about25 g/10 min and a density of 0.966 kg/dm³.

The second reactor was operated under polymerization conditions of about80° C., and a pressure of 2.5 MPa. 14 kg/h of ethylene and 0.75 kg/h of1-hexene were introduced in the downcomer 33 of the second reactor vialine 46. 5 kg/h of propane, 28.5 kg/h of ethylene and 3.1 g/h ofhydrogen were fed through line 45 into the recycling system.

In order to broaden the molecular weight distribution of the finalethylene polymer, the second reactor was operated by establishingdifferent conditions of monomers and hydrogen concentration within theriser 32 and the downcomer 33. This is achieved by feeding via line 52,330 kg/h of a liquid stream (liquid barrier) into the upper part of thedowncomer 33. Said liquid stream has a composition different from thatof the gas mixture present in the riser. Said different concentrationsof monomers and hydrogen within the riser, the downcomer of the secondreactor and the composition of the liquid barrier are indicated inTable 1. The liquid stream of line 52 comes from the condensation stepin the condenser 49, at working conditions of 52° C. and 2.5 MPa,wherein a part of the recycle stream is cooled and partially condensed.As shown in the FIGURE, a separating vessel and a pump are placed, inthe order, downstream the condenser 49. The final polymer wasdiscontinuously discharged via line 54.

The polymerization process in the second reactor produced relativelyhigh molecular weight polyethylene fractions. In Table 1 the propertiesof the final product are specified. It can be to seen that the meltindex of the final product is decreased as compared to the ethyleneresin produced in the first reactor, showing the formation of highmolecular weight fractions in the second reactor.

The first reactor produced around 44.5% by weight (split wt %) of thetotal amount of the final polyethylene resin produced by both first andsecond reactors. At the same time, the obtained polymer is endowed witha relatively broad molecular weight distribution as witnessed by a ratioMIF/MIP equal to 23.7.

Example 2

The process of the invention was carried out with the same setup and thesame polymerization catalyst of Example 1. Also the process conditionsand consequently the obtained polymer properties of the first reactorwere the same.

The second reactor was operated under polymerization conditions of about80° C., and a pressure of 2.5 MPa. 14 kg/h of ethylene and 0.86 kg/h of1-hexene were introduced in the downcomer of the second reactor via line46. 5 kg/h of propane, 27.4 kg/h of ethylene and 3.6 g/h of hydrogenwere fed through line 45 into the recycling system.

In order to broaden again the molecular weight distribution of the finalethylene polymer, the second reactor was operated by establishingdifferent conditions of monomers and hydrogen concentration within theriser 32 and the downcomer 33. Again 330 kg/h of barrier liquid were fedvia line 52. The gas compositions of the riser, the downcomer and theliquid barrier are indicated in Table 1. The liquid stream of line 52comes from the condensation step in the condenser 49, at workingconditions of 51° C. and 2.5 MPa, wherein a part of the recycle streamis cooled and partially condensed.

The first reactor produced around 45% by weight (split wt %) of thetotal amount of the final polyethylene resin produced by both first andsecond reactors. At the same time, the obtained polymer is endowed witha relatively broad molecular weight distribution as witnessed by a ratioMIF/MIP equal to 22.5.

Comparative Example 1

The polymerization was carried out using the same setup of Examples 1and 2, but the polymerization catalyst was the same as used in example 6of WO2005019280.

8 g/h of the solid catalyst component prepared as described above werefed, using 5 kg/h of liquid propane, to a precontacting apparatus, inwhich triethylaluminum (TEA) as well tetrahydrofuran (THF) were dosed.The weight ratio between aluminum alkyl and solid catalyst component was5:1. The weight ratio between aluminum alkyl and THF was 44. Theprecontacting step was carried out under stirring at 50° C. with a totalresidence time of 70 minutes.

The catalyst enters the first gas-phase polymerization reactor 1 of theFIGURE via line 10. In the first reactor ethylene was polymerized usingH₂ as molecular weight regulator and in the presence of propane as inertdiluent. 40 kg/h of ethylene and 75 g/h of hydrogen were fed to thefirst reactor via line 9. No comonomer was fed to the first reactor.

The polymerization was carried out at a temperature of 80° C. and at apressure of 2.4 MPa. The polymer obtained in the first reactor wasdiscontinuously discharged via line 11, separated from the gas into thegas/solid separator 12, and reintroduced into the second gas-phasereactor via line 14.

The polymer produced in the first reactor had a melt index MIE of about100 g/10 min and a density of 0.968 kg/dm³.

The second reactor was operated under polymerization conditions of about80° C., and a pressure of 2.1 MPa. 12 kg/h of ethylene and 1.5 kg/h of1-hexene were introduced in the downcomer 33 of the second reactor vialine 46. 5 kg/h of propane, 26.5 kg/h of ethylene and 1.2 g/h ofhydrogen were fed through line 45 into the recycling system.

In order to broaden the molecular weight distribution of the finalethylene polymer, the second reactor was operated by establishingdifferent conditions of monomers and hydrogen concentration within theriser 32 and the downcomer 33. This is achieved by feeding via line 52,200 kg/h of a liquid stream (liquid barrier) into the upper part of thedowncomer 33. Said liquid stream has a composition different from thatof the gas mixture present in the riser. Said different concentrationsof monomers and hydrogen within the riser, the downcomer of the secondreactor and the composition of the liquid barrier are indicated inTable 1. The liquid stream of line 52 comes from the condensation stepin the condenser 49, at working conditions of 53° C. and 2.1 MPa,wherein a part of the recycle stream is cooled and partially condensed.As shown in the FIGURE, a separating vessel and a pump are placed, inthe order, downstream the condenser 49. The final polymer wasdiscontinuously discharged via line 54.

The polymerization process in the second reactor produced relativelyhigh molecular weight polyethylene fractions. In Table 1 the propertiesof the final product are specified. It can be seen that the melt indexof the final product is decreased as compared to the ethylene resinproduced in the first reactor, showing the formation of high molecularweight fractions in the second reactor.

The first reactor produced around 50% by weight (split wt %) of thetotal amount of the final polyethylene resin produced by both first andsecond reactors. At the same time, the obtained polymer is endowed witha relatively broad molecular weight distribution as witnessed by a ratioMIF/MIP equal to 38.8

Comparative Example 2

The polymer of this comparative example is a prior-art polyethylenecomposition prepared with a Cr-catalyst, in a single gas-phase reactor.

TABLE 1 Ex. 1 Ex. 2 Comp. 1 Comp. 2 Operative conditions first reactorH₂/C₂H₄ Molar ratio 1.9 1.9 1.7 C₂H₄% 12.1 12.4 14 Split (wt %) 44.5 4550 Operative conditions second reactor H₂/C₂H₄ Molar ratio riser 0.1570.203 0.038 C₂H₄% riser 11.3 11.4 15 C₆H₁₂ riser 0.56 0.65 1.2 H₂/C₂H₄Molar ratio downcomer 0.069 0.086 0.04 C₂H₄% downcomer 2.6 2.8 5.4 C₆H₁₂downcomer 0.60 0.71 2.2 H₂/C₂H₄ Molar ratio barrier 0.013 0.015 0.01C₂H₄% barrier 6.8 7.1 6.5 C₆H₁₂ barrier 0.93 1.17 2.7 Final Polymerproperties MIP [5 kg] (g/10 min.) 0.2 0.29 0.21 0.31 MIF [21.6 kg] (g/10min.) 4.8 6.5 8.15 6.25 MIF/MIP 23.7 22.5 38.8 20.16 Density (kg/dm³)0.9509 0.9496 0.9487 0.947 Mw [g/mol] 3.5E+5 3.8E+5 3.6E+5 3.9E+5 Mz[g/mol] 2.0E+6 8.3E+6 5.0E+6 3.5E+6 Mw/Mn 25 27 52 25 LCB 0.89 0.84 0.690.99 Comonomer content IR 0.7% ± 0.1 0.7% ± 0.1 1.6 1.6 [% by weight](C₆H₁₂) (C₆H₁₂) (C₆H₁₂) (C₆H₁₂) SIC index 3.8 3.3 1.9 6.1 Swell ratio(%) 179 171 120 210 Shrinkage@1500 s⁻¹, 15 12 — 23 T = 190° C. [%]Critical shear-rate for sharkskin, 300 300 — 200 T = 190° C., [1/s]Notched-Tensile Impact test, 164 155 93 145 T = −30° C. [kJ/m²] FNCT 4MPa/80° C. (hours)* 329 20 >2000 4 Charpy aFM, T = 0° C. [kJ/m²] — — 8.9Notes: C₂H₄ = ethylene; C₆H₁₂ = hexene; *aqueous solution of 2% ArkopalN100

What is claimed is:
 1. A manufactured article comprising a polyethylenecomposition having the following properties: 1) a density from 0.945 toless than 0.952 g/cm³, determined according to ISO 1183 at 23° C.; 2) aMIF/MIP ratio from 15 to 30, wherein the MIF is the melt flow index at190° C. with a load of 21.60 kg, and the MIP is the melt flow index at190° C. with a load of 5 kg, both determined according to ISO 1133; and3) a SIC Index from 2.5 to 5.5; wherein the SIC Index is theShear-Induced Crystallization Index as determined according to thefollowing relation:SIC Index=(t _(onset,SIC)@1000×t _(onset,quiescent))/(HLMI)*100 wheret_(onset,SIC)@1000 is measured in seconds and is the time required forcrystallization onset under a shear rate of 1000 s⁻¹, thet_(onset, quiescent) is measured in seconds and is the crystallizationonset time at a temperature of 125° C. under no shear, as determined inisothermal mode by differential scanning calorimetry; and HLMI is themelt flow index determined at 190° C. with a load of 21.6 kg, accordingto ISO
 1133. 2. The manufactured article of claim 1, comprising one ormore ethylene copolymers.
 3. The manufactured article of claim 2,wherein the one or more ethylene copolymers have a comonomer contentequal to or less than 1% by weight.
 4. The manufactured article of claim2, wherein the comonomer is selected from olefins having the generalformula CH₂═CHR, wherein R is a linear or branched alkyl radical havingfrom 1 to 10 carbon atoms.
 5. The polyethylene composition of claim 1,obtained by using a Ziegler-Natta polymerization catalyst.
 6. Themanufactured article of claim 1, having at least one of the followingproperties: a Mw equal to or greater than 250000 g/mol; a Mw/Mn from 20to 30; a Long Chain Branching index equal to or greater than 0.70; a MIPfrom 0.05-0.5 g/10 min; and a MIF from 1-15 g/10 min.
 7. Themanufactured article of claim 1, comprising: A) 30-50% by weight of anethylene homopolymer or copolymer with a density equal to or greaterthan 0.960 g/cm³ and a melt flow index (MIE) at 190° C. with a load of2.16 kg, according to ISO 1133, of 10-35 g/10 min; and B) 50-70% byweight of an ethylene copolymer having a MIE value lower than the MIEvalue of A).
 8. The manufactured article of claim 1, comprising ablow-molded article.
 9. The manufactured article of claim 1, wherein thepolyethylene composition is formed in one or more polymerizing steps,and wherein all of the polymerization steps are carried out in thepresence of a Ziegler-Natta polymerization catalyst supported on MgCl₂.10. The manufactured article of claim 9, wherein the one or morepolymerizing steps comprise the following steps, in any mutual order: a)polymerizing ethylene, optionally together with one or more comonomers,in a gas-phase reactor in the presence of hydrogen; b) copolymerizingethylene with one or more comonomers in another gas-phase reactor in thepresence of an amount of hydrogen less than step a); wherein at leastone of said gas-phase reactors the growing polymer particles flow upwardthrough a first polymerization zone under fast fluidization or transportconditions, leave said riser and enter a second polymerization zonethrough which they flow downward under the action of gravity, leave saidsecond polymerization zone and are reintroduced into the firstpolymerization zone for establishing a circulation of polymer betweensaid two polymerization zones.
 11. The manufactured article of claim 8,wherein the blow-molded article is selected from an open top drum (OTD),an industrial bulk article (IBC), a container and a gasoline storagetank.